I would like to control 1 light from multiple places (3 and more switches) and I am thinking about to add additional "pseudo" 3-way switch at beginning of chain which will be replaced by UniPi relay. Switches in the middle will be 4-way (switch #7).

I am aware, that I wound be able to monitor, whether the light is really on or off. Or is there any way?

@blackdot I do not think so. The other way might be to use the wall switches as push buttons and connect them to 12VDC to digital input and do the switching logic and switch the light only from one place = relay of UniPi so you would have the full control on it

I don't want to give up the classical switches, which operates on 220V.
In this way the each light will be almost independent on UniPi, which could be installed in house later.
(Only each relay of UniPi will be replaced by hard-connected cables to simulate 3-way switch)